(1) Field of the Invention
The present invention relates to a nitride semiconductor material, a semiconductor element, and a manufacturing method thereof. In particular, the present invention relates to a nitride semiconductor material and a semiconductor element which are made of a group III nitride semiconductor and a group IV nitride, and a manufacturing method thereof.
(2) Description of the Related Art
Recently, research and development of an electronic device such as a transistor made of a group III nitride semiconductor, for example, GaN is being widely carried out. Since a band gap of GaN is large, and a saturated drift velocity of the electron of GaN is high, GaN offers much promise for high power high frequency electron devices. For example, a semiconductor element that uses GaN is described in Japanese Laid-Open Patent Application No. 2004-228481. Moreover, it is reported that it is possible to obtain a large value of 152 GHz for a frequency cutoff fT in a high power high frequency electron device that uses GaN (refer to M. Higashiwaki, N. Hirose and T. Matsui, “Cat-CVD SiN-Passivated AlGaN-GaN HFETs with Thin and High Al Composition Barrier Layers”, IEEE Electron Device Letters 26, 139, 2005, and M. Higashiwaki and T. Matsui, “AlGaN/GaN Heterostructure Field-Effect Transistors with Current Gain Cut-off Frequency of 152 GHz on Sapphire Substrates”, Japanese Journal of Applied Physics 44, L475, 2005).
According to the above-mentioned two non-patent documents by M. Higashiwaki et al, by depositing a SiN film on a heterojunction of 10 nm thickness Al0.4Ga0.6N/1.3 nm thickness AlN/1.5 μm thickness using Cat-CVD (Catalytic Chemical Vapor Deposition), it is possible to increase sheet carrier density to 2.5×1013 cm−2 which is nearly three times that compared with a heterojunction when there is no SiN film. As a result, by greatly reducing sheet resistance between a source and a drain electrode and shortening the gate length to about 100 nm or less, it is possible to increase transconductance corresponding to the gain of a transistor to 400 mS/mm or more.
As mentioned above, a GaN-based transistor having excellent high frequency characteristics can be realized by passivating a thin film SiN on a GaN-based heterojunction transistor, forming a gate electrode thereon, and adopting an MIS (Metal-Insulator-Semiconductor) type transistor.